Choosing an Appropriate Pull-up/Pull-down Resistor for ...

[Pages:9]Application Report

SLVA485 ? October 2011

Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs

Ben Hopf ....................................................................................... PMP-DC/DC Low-Power Converters

ABSTRACT

Many ICs contain digital output pins to indicate certain statuses to the rest of the system. These outputs fall into two categories: open drain (open collector for bipolar outputs) or push-pull (also known as totem pole). Open drain outputs are commonly utilized because they offer several advantages when compared to push-pull outputs. Unlike push-pull outputs, several open drain outputs from different devices can be connected directly together to create an OR function. Also, open drain outputs provide more flexibility to a designer as they can be pulled-up to any voltage found in the system, which can be useful when they serve as inputs to a processor which might require a lower voltage level than the push-pull output would give. Examples of open drain outputs commonly found on ICs include Power Good (PG) and Low Battery (LBO) on switching regulators, reset and Power Fail (PFO) on supply voltage supervisors (SVS), and Low Battery, Power Fail, and reset on power management units. All open drain outputs require the use of an external pull-up or pull-down resistor to keep the digital output in a defined logic state. This application report discusses when to use a pull-up or pull-down resistor, the factors that should be considered when selecting a pull-up or pull-down resistor, and how to calculate a valid range for the value of the resistor.

Contents

1 Introduction .................................................................................................................. 2 2 Calculating the Pull-up Resistor Range .................................................................................. 2 3 Calculating the Range of RPull-up for the TPS62067 ..................................................................... 4 4 Calculating the Pull-down Resistor Range .............................................................................. 5 5 Calculating the Range of RPull-down for the TL7759 ....................................................................... 6 6 Other Selection Considerations ........................................................................................... 7 7 Conclusion ................................................................................................................... 8

List of Figures

1 Typical PG Output Equivalent Circuit (PG Floating High) ............................................................. 2 2 Typical PG Output Equivalent Circuit (PG Low) ........................................................................ 3 3 Reset Output Equivalent Circuit (Reset Floating Low)................................................................. 5 4 Reset Output Equivalent Circuit (Reset High)........................................................................... 6

List of Tables

1 PG Output IC ............................................................................................................... 4 2 EN Input IC .................................................................................................................. 4 3 Reset Output IC ............................................................................................................ 6 4 Typical Reset Input IC ..................................................................................................... 7

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Introduction



1 Introduction

An SVS monitors a critical voltage within a system and outputs a reset signal if that voltage drops below a specified threshold. This reset signal is the open drain output in need of a pull-up or pull-down resistor. An SVS can also have an open drain PFO output that asserts if some voltage in the system drops below a specified threshold.

In a power converter, a PG output is routinely used to drive the enable input of a subsequent IC. A PG output is low if the chip's output voltage is below a certain percentage of its nominal value. The PG output is then pulled high by the external pull-up resistor when the voltage has reached the specified level. Power converters can also contain LBO outputs that are asserted if the LBI pin voltage drops below a specified threshold.

The first thing to recognize when dealing with an open drain output is whether a pull-up or a pull-down resistor is needed. This depends on whether the IC drives the output high or low when it wants to assert it. For example, the TPS62067 step-down converter has a PG output that it drives low if the chip's output voltage is not in regulation. Therefore, PG needs a pull-up resistor that pulls the PG pin high when the chip allows it to float, indicating that power is good. On the other hand, the TL7759 SVS has a reset output that it drives high when it detects a supply voltage drop below a specific threshold. This reset output needs a pull-down resistor to pull it low when that voltage is above the specified threshold.

To determine the value of the pull-up or pull-down resistor, several factors need to be taken into consideration. These include the output pin's leakage current, the leakage current of the input pin that the open drain output is connected to, the voltage that the output is being pulled up to, the high or low voltage logic level of both the output pin and the input pin that the output is connected to, and the test current used to obtain the high or low voltage logic level. This report analyzes one open drain output and one open collector output. The first example features the TPS62067 step down converter and demonstrates when a pull-up resistor is needed and how to calculate a range for it. The second example uses the TL7759 SVS and demonstrates when a pull-down resistor is needed and how to calculate a range of values for it.

2 Calculating the Pull-up Resistor Range

Figure 1 shows typical PG output circuitry. The PG output connects to the output voltage, Vout, through a pull-up resistor, RPull-up, and then connects to the EN input of another chip.

VOUT

Inside the IC

IPull-up

RPull-up

Comparator + _

ILKG

EN PG VPG IEN

Q1 (OFF)

Figure 1. Typical PG Output Equivalent Circuit (PG Floating High)

The circuit in Figure 1 is analyzed to find the maximum value for RPull-up, when power is good and Q1 is off. Although Q1 is off, the datasheet specifies that there is some leakage current through it. This value is found in the datasheet as ILKG, the leakage current into the PG pin. This leakage current creates a voltage drop across the pull-up resistor. Thus, the voltage on the PG pin and on the subsequent EN input is less

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Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs

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Calculating the Pull-up Resistor Range

than Vout. For the calculation of the maximum value of RPull-up, the maximum value of ILKG is used because it would result in the largest voltage drop across RPull-up. Also, assuming that the PG output feeds the EN input on another chip, there will also be a current flowing into that EN input. This value is found in the

datasheet of the subsequent part and is labeled IEN in Figure 1. Equation 1 shows how to calculate IPull-up using Kirchhoff's Law.

(1)

To calculate the maximum value of the pull-up resistor, Equation 2 sets the voltage at the PG pin, VPG, equal to the subsequent chip's EN pin's VIH. VIH is the minimum voltage that is specified to be read as a logic high.

(2)

This value is a maximum because choosing a larger resistor would result in a larger voltage drop across RPull-up which would cause VPG to be lower than the minimum value of VIH. In other words, the subsequent chip would not recognize the PG voltage as being a logic high.

Figure 2 shows the same circuit as analyzed above when Q1 is on and PG is low. This indicates that the output voltage is below regulation and power is not good.

VOUT

Inside the IC

IPull-up

RPull-up

Comparator + _

EN

IOL

PG VPG IEN

Q1 (ON)

Figure 2. Typical PG Output Equivalent Circuit (PG Low)

When finding the minimum value for RPull-up, it is assumed that Q1 is turned on as shown in Figure 2, so VPG is shorted to ground. In reality, Q1 has a resistance, RDSon, which will drop some voltage and cause the PG voltage to be above ground potential. When Q1 is on, the PG voltage must be sufficiently low to register as a logic low to subsequent circuitry. To calculate the current in the pull-up resistor, the current labeled IOL is needed. This value is found in the IC datasheet as the test current for VOL, the output low voltage level. Setting the maximum current through Q1 equal to the current used in the test condition in the datasheet gives known performance, meaning VOL will not exceed its specified maximum voltage at that current. Currents up to the specified absolute maximum PG sink current can be used, but they could yield a VOL higher than its specified maximum. (In the case of the TPS62067 used in the example, its VOL test current and its absolute maximum PG sink current are equal, but this is not always the case.) Equation 3 uses Kirchhoff's Law, the IC's test current, and the leakage current of the subsequent EN input to calculate the current through the pull-up resistor.

(3)

The voltage across the pull-up resistor is equal to Vout minus VPG. The datasheet gives a maximum value for VPG (called VOL and typically 0.3 V or 0.4 V), but it could be 0 V which would result in a higher current flowing through the pull-up resistor than if it were at the maximum specified voltage. Based on this, Equation 4 calculates the minimum pull-up resistance.

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(4)

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Calculating the Range of RPull-up for the TPS62067



This calculation results in a minimum value because choosing any value lower for RPull-up causes a higher current than the test condition current to flow in Q1. If a current higher than the test condition current flows through Q1, the voltage drop across Q1 is higher and no longer ensured. The results of Equation 4 and Equation 2 yield a range of acceptable values for the pull-up resistor RPull-up at the output of the PG pin.

In the case where PG is low, the maximum allowed VPG voltage depends on whether VOL is greater than or less than VIL, the subsequent EN pin's maximum low-level input voltage. If VOL is greater than VIL, then specified performance is not possible, because the maximum allowed voltage, VIL, is less than the ensured highest voltage on the PG pin, VOL. In this case, the minimum resistor value calculated in Equation 4 should be significantly increased to maintain plenty of margin in the PG voltage in order to achieve a sufficiently low voltage at the PG output to register as a logic low with subsequent circuitry. Since specified performance is desired, these calculations assume that VOL is less than VIL.

3 Calculating the Range of RPull-up for the TPS62067

Table 1 gives values from the PG output IC's datasheet (in this case the TPS62067 [SLVS833A]), and Table 2 gives values for the EN input from that IC's datasheet (also a TPS62067 [SLVS833A]) that are used to calculate a range of values for the pull-up resistor at the PG output. For this example, the TPS62067's PG output drives the EN input of another TPS62067 chip.

Table 1. PG Output IC

TPS62067 DATASHEET VALUES

PARAMETER

VALUE

IC

TPS62067

Pin

Power Good (PG)

IOL ILKG(max) VOL(max)

1 mA 100 nA 0.3 V

Table 2. EN Input IC

TPS62067 DATASHEET VALUES

PARAMETER

VALUE

IC

TPS62067

Pin

Enable (EN)

VIH(min) IEN(max) VIL(max)

1.0 V 1000 nA

0.4 V

First, use IEN, ILKG, and Equation 1 to find the current through the pull-up resistor, IPull-up.

(5)

Now that IPull-up has been calculated, the maximum value for RPull-up is found by using Equation 2. An output voltage, Vout, of 1.8 V is used as an example.

(6) The next step is to find the minimum pull-up resistor value. Equation 3 is utilized to find the current through the pull-up resistor.

(7) With this value, Equation 4 finds the minimum value for RPull-up.

(8)

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With this final calculation, the range of pull-up resistor values is

Calculating the Pull-down Resistor Range (9)

4 Calculating the Pull-down Resistor Range

Figure 3 shows an IC's reset output connected to ground through a pull-down resistor, RPull-down. The reset pin is then connected to the reset input of a microcontroller or microprocessor.

Inside the IC

Comparator + _

VCC

Q1 (OFF)

IOL

RESET VRESET

IR

uP

reset

IPDL

RPull-down

Figure 3. Reset Output Equivalent Circuit (Reset Floating Low)

The circuit in Figure 3 is analyzed to find the maximum value for RPull-down, when reset is low and Q1 is off. Although Q1 is off, the datasheet specifies that there is some leakage current through it. This value is found in the datasheet as IOL, the low-level output current for reset, and it creates a voltage drop across the pull-down resistor. Thus, the voltage on the reset output pin and the subsequent reset input pin is greater than zero volts. There is also current flowing out of the processor's reset input, and this value is found in the datasheet of the processor as the reset input leakage current, labeled IR in Figure 3. Equation 10 uses Kirchhoff's Law to calculate the current through the pull-down resistor, IPDL, when reset is pulled down.

(10)

VIL is found in the datasheet for the processor as the maximum low-level input voltage at its reset pin. This value represents the maximum voltage that results in the processor recognizing the reset signal as logic low. Equation 11 implements Ohm's Law to calculate the maximum value for Rpull-down.

(11)

Figure 4 shows the same circuit as analyzed in Figure 3 when Q1 is on and reset is high. This indicates that the voltage being monitored by the supervisor is below the specified threshold, and the processor needs to be in reset.

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Calculating the Range of RPull-down for the TL7759

Inside the IC

Comparator

+ _

VCC

Q1 (ON)

IOH

RESET VRESET

IR

uP

reset

IPDH

RPull-down



Figure 4. Reset Output Equivalent Circuit (Reset High)

Using Figure 4 to find the minimum value for RPull-down, it is assumed that Q1 is turned on so that VRESET is shorted to VCC. In reality, Q1 has a saturation voltage that will cause VRESET to be lower than VCC. VRESET must be high enough to be read as a logic high by the connected circuitry's input. The IC datasheet gives a value for the reset high-level output voltage, VOH, at some test current IOH. This gives known performance, meaning VOH will not fall below its specified minimum. Currents up to the specified absolute maximum IOH can be used, but they could yield a VOH lower than its specified minimum. Equation 12 uses Kirchhoff's Law, the IC's test current, and the leakage current of the subsequent reset input to calculate the current through the pull-down resistor.

(12)

After calculating the value for IPDH, Equation 13 is used to find the minimum pull-down resistor value.

(13)

In the case where reset is high, the minimum allowed VRESET voltage depends on whether VOH is greater than or less than VIH, the subsequent reset input pin's minimum high-level input voltage. If VOH is lower than VIH, then specified performance is not possible, because the minimum required voltage, VIH, is greater than the specified lowest voltage on the reset pin, VOH. In this case, the minimum resistor value calculated in Equation 13 should be significantly increased to maintain plenty of margin in the reset voltage in order to achieve a sufficiently high voltage at the reset output to register as a logic high with the subsequent circuitry. Since specified performance is desired, these calculations assume that VOH is greater than VIH.

5 Calculating the Range of RPull-down for the TL7759

Table 3 gives values from the reset output IC's datasheet (in this case the TL7759), while Table 4 gives typical values for IR and VIL that are found in the reset input IC's datasheet. The values from both of these tables are used to calculate a range of values for the pull-down resistor at the reset output.

Table 3. Reset Output IC

TL7759 DATASHEET VALUES

PARAMETER

VALUE

IC

TL7759

Pin

Reset (Output)

VCC IOL(max) IOH(min)

IOH

5V 1 ?A 4V 8 mA

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Table 4. Typical Reset Input IC

DATASHEET VALUES

PARAMETER

VALUE

Pin

Reset (Input)

VIL(max) IR(max) VIH(min)

0.3 V 100 nA

3V

Other Selection Considerations

First, use IOL, IR, and IPDL as well as Equation 10 to find a value for IPDL. (14)

Now use Equation 11 to calculate the maximum value for the pull-down resistor.

(15) The next step is to use Equation 12 to find IPDH.

(16) Lastly, Equation 13 is utilized to find the minimum pull-down resistor value.

(17)

This final calculation yields a range for the pull-down resistor of

(18)

6 Other Selection Considerations

The above examples are calculated using parameters from the datasheet that ensure performance, such as the output low voltage at a certain test current. If the current sunk by the PG pin is lower than the test condition current, then the voltage drop across the PG pin's FET is lower and the output low voltage is lower. If however, the current sunk by the PG pin is higher than the test condition current, then the voltage at the PG pin could be higher than specified, leading to an unreadable logic voltage level. Therefore, the test condition current is always used for the maximum allowed current in the above calculations because it specifies a maximum voltage at the output pin. Currents up to the absolute maximum rating of the output pin could be used, but then the output voltage at the PG or reset pin is no longer specified to fall in its specified range. To approximate the output voltage at the PG pin for currents higher than the test condition current, first the on resistance, or RDSon, of the internal FET is calculated by using the specified maximum PG output-low voltage, VOL, and its test current, IOL.

(19)

Then the output voltage at the PG pin is just RDSon times the new sink current.

(20)

This voltage should remain below VIL in order to be read as a logic low. Alternatively, a graph of the PG or reset voltage versus current may be shown in the datasheet. These graphs show typical performance and, like the above RDSon calculation, are not specified performance measures.

After establishing the range for the pull-up or pull-down resistor, there are other factors to consider when selecting a resistor that falls within the established range. One factor that discourages using a resistor near the low end of the range is the power consumption through the pull-up or pull-down resistor and the drive circuitry. For example, if the minimum pull-down resistor of 500 is used and the saturation voltage

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Conclusion



of the BJT happens to be the maximum value of 1 V, the pull-down resistor drops VCC ? 1 V across it. If VCC is 5 V, the 4 V drop across RPull-down produces 8 mA of current and a resulting 32 mW power loss in the pull-down resistor plus an additional 8 mW in the IC's driver BJT. This total power loss, VCC times Isink, might be very significant for some applications, in which case a larger pull-down resistor should be selected.

However, there are disadvantages to selecting the largest allowed pull-up resistor. Larger resistances create a higher impedance net which is more susceptible to picking up noise from other nearby signals on the board that happen to couple to it. This is especially a concern for lengthy open drain outputs routed over a long distance.

A second concern for large resistor values is from parasitic capacitance on the open drain output. This can come from other open drain outputs OR'd together, from the downstream input pins, or from nearby traces that create a capacitor in the board. This high capacitance creates an RC circuit that has an associated rise time and fall time. Final circuit operation should be validated with these longer than normal rise and fall times.

7 Conclusion

Open drain outputs found on power devices require pull-up or pull-down resistors to keep the digital output in a defined logic state. An acceptable range of values for this resistor is calculated using circuit analysis and some parameters from the part's datasheet. Choosing an appropriate resistor value within this range ensures that the output is correctly recognized by the subsequent chip's input pin. This range of acceptable values provides flexibility in the actual value selected, which allows, for example, the reuse of a resistor value already in the Bill of Materials. The methods and equations presented in this application note can be utilized to find an appropriate range of resistor values and allows the designer to select the value that best fits the application.

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